LaTeX/TeX. I'd written one maths paper before using LaTeX, and used QuickLaTeX on my website for a while, but early this year I read or looked at almost every available book on LaTeX and TeX. (There aren't that many) Plus consulted TeX StackExchange a lot every day for a while, which has most of the people who wrote the best books and packages answering questions, it's amazing. Also got better at using TikZ within LaTeX. (It makes doing diagrams pretty easy) It took a few months to get on top of LaTeX and the ~20 packages I mostly use. (I'm no plain TeX master, but can at least start to recognize what code in TeX is doing, and have some understanding how TeX works.) Since then I do all my writing in LaTeX (I use TexShop) - writing books, diary, notes...anything I learn about, I make a lovely-looking book on it while I go. (Before that I just had disorganized notes on paper I never looked at again.) I have a book for my programming, at last keeping the extensive notes on everything I always should have. I wish I'd done this a long time ago.